Cortex-M backend: Add cmsis_nn pybind dependency (#19279)

- Add cmsis_nn to new optional-dependency called [cortex_m]
    
    - Additionally, add an install step that allows optional
    depedencies to install required build tools. This is needed
    for cortex_m.
    
    - Add cmsis_nn to backends/cortex_m/requirements-cortex-m.txt
    and add possibility to install from setup.sh to not break
    backwards compatability, and to mirror ethos_u and vgf backend.
    
    - Add guard to cortex_m/passes/__init__.py to fail gracefully if
    cortex_m dependencies are not installed.
    
Signed-off-by: Erik Lundell <erik.lundell@arm.com>

Author: Erik-Lundell

backends/cortex_m/CMakeLists.txt

@@ -23,7 +23,7 @@ include(FetchContent)
 
 # CMSIS-NN configuration with dynamic path detection
 set(CMSIS_NN_VERSION
-    "098d54a61e3e04e2b60e9010e871eef036ac5ae3"
+    "d933672e7ca97eec70ef43230baee7b20c2a28ae"
     CACHE STRING "CMSIS-NN version to download"
 )
 set(CMSIS_NN_LOCAL_PATH

backends/cortex_m/passes/__init__.py

@@ -3,6 +3,36 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
+from importlib.util import find_spec
+
+
+def _missing_dependencies_error(missing: str) -> ModuleNotFoundError:
+    return ModuleNotFoundError(
+        "Cortex-M backend dependencies are not installed "
+        f"(missing: {missing}). Install ExecuTorch with "
+        "`pip install executorch[cortex_m]`, or if building from source run "
+        "`examples/arm/setup.sh --i-agree-to-the-contained-eula`."
+    )
+
+
+def _ensure_cortex_m_dependencies() -> None:
+    required_modules = {
+        "cmsis_nn": "cmsis_nn",
+    }
+    missing_packages = []
+    for module_name, package_name in required_modules.items():
+        try:
+            if find_spec(module_name) is None:
+                missing_packages.append(package_name)
+        except (ImportError, ValueError):
+            missing_packages.append(package_name)
+
+    if missing_packages:
+        raise _missing_dependencies_error(", ".join(missing_packages))
+
+
+_ensure_cortex_m_dependencies()
+
 from .activation_fusion_pass import ActivationFusionPass  # noqa
 from .clamp_hardswish_pass import ClampHardswishPass  # noqa
 from .convert_to_cortex_m_pass import ConvertToCortexMPass  # noqa

backends/cortex_m/requirements-cortex-m.txt

@@ -0,0 +1,8 @@
+# Copyright 2026 Arm Limited and/or its affiliates.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+# These dependencies need to match pyproject.toml
+
+cmsis_nn @ git+https://github.com/ARM-software/CMSIS-NN.git@d933672e7ca97eec70ef43230baee7b20c2a28ae

backends/cortex_m/test/misc/test_cmsis_pybind.py

@@ -0,0 +1,33 @@
+# Copyright 2026 Arm Limited and/or its affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import importlib
+
+import pytest
+
+
+def _import_cmsis_nn():
+    try:
+        return importlib.import_module("cmsis_nn")
+    except Exception as exc:
+        pytest.fail(f"Failed to resolve cmsis_nn: {exc}")
+
+
+def test_cmsis_nn_convolve_wrapper_buffer_size() -> None:
+    cmsis_nn = _import_cmsis_nn()
+
+    buf_size = cmsis_nn.convolve_wrapper_buffer_size(
+        cmsis_nn.Backend.MVE,
+        cmsis_nn.DataType.A8W8,
+        input_nhwc=[1, 8, 8, 16],
+        filter_nhwc=[8, 3, 3, 16],
+        output_nhwc=[1, 6, 6, 8],
+        padding_hw=[0, 0],
+        stride_hw=[1, 1],
+        dilation_hw=[1, 1],
+    )
+
+    assert buf_size == 576

examples/arm/setup.sh

@@ -22,6 +22,7 @@ enable_baremetal_toolchain=1
 target_toolchain=""
 enable_fvps=1
 enable_vela=1
+enable_cortex_m=1
 enable_model_converter=0   # model-converter tool for VGF output
 enable_vgf_lib=0  # vgf reader - runtime backend dependency
 enable_emulation_layer=0  # Vulkan layer driver - emulates Vulkan ML extensions
@@ -48,6 +49,7 @@ OPTION_LIST=(
   "--target-toolchain Select toolchain: gnu (default), zephyr, or linux-musl"
   "--enable-fvps Enable FVP setup"
   "--enable-vela Enable VELA setup"
+  "--disable-cortex-m-deps Do not setup what is needed for Cortex-M"
   "--enable-model-converter Enable MLSDK model converter setup"
   "--enable-vgf-lib Enable MLSDK vgf library setup"
   "--enable-emulation-layer Enable MLSDK Vulkan emulation layer"
@@ -123,6 +125,10 @@ function check_options() {
                 enable_vela=1
                 shift
                 ;;
+            --disable-cortex-m-deps)
+                enable_cortex_m=0
+                shift
+                ;;
             --enable-model-converter)
                 enable_model_converter=1
                 shift
@@ -211,6 +217,11 @@ function setup_ethos_u_tools() {
     CMAKE_POLICY_VERSION_MINIMUM=3.5 BUILD_PYBIND=1 pip install --no-dependencies -r $et_dir/backends/arm/requirements-arm-ethos-u.txt
 }
 
+function setup_cortex_m_tools() {
+    log_step "cortex-m-tools" "Installing Cortex-M Python tooling"
+    pip install --no-dependencies -r $et_dir/backends/cortex_m/requirements-cortex-m.txt
+}
+
 function setup_mlsdk_dependencies() {
     log_step "mlsdk" "Installing MLSDK dependencies"
     pip install -r $et_dir/backends/arm/requirements-arm-vgf.txt
@@ -299,6 +310,7 @@ if [[ $is_script_sourced -eq 0 ]]; then
              "root=${root_dir}, target-toolchain=${target_toolchain:-<default>}"
     log_step "options" \
              "ethos-u: fvps=${enable_fvps}, toolchain=${enable_baremetal_toolchain}, vela=${enable_vela} | " \
+             "cortex-m: deps=${enable_cortex_m} | " \
              "mlsdk: model-converter=${enable_model_converter}, vgf-lib=${enable_vgf_lib}, " \
                     "emu-layer=${enable_emulation_layer}, vulkan-sdk=${enable_vulkan_sdk}"
 
@@ -349,6 +361,11 @@ if [[ $is_script_sourced -eq 0 ]]; then
         setup_ethos_u_tools
     fi
 
+    if [[ "${enable_cortex_m}" -eq 1 ]]; then
+        log_step "deps" "Installing Cortex-M CMSIS-NN tooling"
+        setup_cortex_m_tools
+    fi
+
     log_step "main" "Setup complete"
     exit 0
 fi

install_executorch.py

@@ -191,7 +191,7 @@ def _parse_args() -> argparse.Namespace:
         help="Only installs necessary dependencies for core executorch and skips "
         " packages necessary for running example scripts.",
     )
-    allowed_optional_dependencies = ["ethos_u", "vgf", "openvino"]
+    allowed_optional_dependencies = ["cortex_m", "ethos_u", "vgf", "openvino"]
     parser.add_argument(
         "--optional-dependency",
         action="append",
@@ -225,7 +225,20 @@ def main(args):
     # Step 1: Install core dependencies first
     install_requirements(use_pytorch_nightly)
 
-    # Step 2: Install core package
+    # Step 2: Install build dependencies for optional dependencies
+    # They need to be installed before optional dependencies due to --no-build-isolation
+    optional_build_dependencies: list[str] = []
+    for optional_dep in args.optional_dependency:
+        match optional_dep:
+            case "cortex_m":
+                optional_build_dependencies.extend(
+                    ["pybind11>=2.10", "scikit-build-core>=0.7"]
+                )
+    if len(optional_build_dependencies) > 0:
+        cmd = [sys.executable, "-m", "pip", "install", *optional_build_dependencies]
+        subprocess.run(cmd, check=True)
+
+    # Step 3: Install core package
     package_spec = "."
     if args.optional_dependency:
         extras = ",".join(dict.fromkeys(args.optional_dependency))
@@ -246,7 +259,7 @@ def main(args):
     )
     subprocess.run(cmd, check=True)
 
-    # Step 3: Extra (optional) packages that is only useful for running examples.
+    # Step 4: Extra (optional) packages that is only useful for running examples.
     if not args.minimal:
         install_optional_example_requirements(use_pytorch_nightly)
 

pyproject.toml

@@ -77,6 +77,10 @@ dependencies=[
 ]
 
 [project.optional-dependencies]
+cortex_m = [
+  # Keep this in sync with AoT deps from backends/cortex_m/requirements-cortex-m.txt
+  "cmsis_nn @ git+https://github.com/ARM-software/CMSIS-NN.git@d933672e7ca97eec70ef43230baee7b20c2a28ae",
+]
 vgf = [
   # AoT vgf dependencies
   # Keep this in sync with AoT deps from backends/arm/requirements-arm-vgf.txt and

requirements-dev.txt

@@ -14,4 +14,4 @@ lintrunner-adapters==0.13.0
 pytest<9.0
 pytest-xdist
 pytest-rerunfailures==15.1
-pytest-json-report
+pytest-json-report